Bug 1061215 - [Messages][Refactoring] Improve handling of remaining chars counter. r=schung

That's why I didn't add it in the previous code :)

But actually there are several ways to look at this:
* updating DOM when the element is display: none is not costful (not free, but quite). So this is probably the simplest solution. However it's still not quite correct if the user adds an image, adds some text, and then remove the image, because we don't retrieve the segmentinfo when there are attachments, and we don't remove the segmentinfo text from the DOM either.
* we can also bind onSegmentInfoChange on the "type" event; should be similar to previous solution, if I don't miss something?
* most correct solution IMO:
** when going from SMS to MMS => empty the counter
** when going from MMS to SMS => wait for the segmentinfo change to add the counter back (means, don't change this code)
** if I don't miss something, it means we'll have the new computed correct counter count after a roundtrip with the gecko API, right?